1. Field of the Invention
This invention relates generally to the field of communications systems, and more particularly to a communication system providing a spectrally efficient digital FM modulation system for use with a conventional frequency modulated (FM) receiver.
2. Description of the Prior Art
A number of problems exist with digital frequency modulated (FM) transmission systems. One such problem relates to splatter and adjacent channel interference created when rectangular pulses, such as found in strings of digital information, is transmitted. It is well known that this splatter is reduced by low pass filtering the baseband modulating signal. Many waveshaping functions are known which minimize the spectrum occupied by the baseband modulating signal. Unfortunately, in an FM system, minimal occupied baseband spectrum does not imply minimal occupied radio frequency (RF) spectrum. Many of the well known waveshaping functions produce high modulation overshoots which result in excessive splatter, even though the baseband frequency spectrum is well contained. A means of reliably minimizing the splatter of the transmitted RF signal, considering both baseband spectrum and modulation overshoots is required.
A second problem occurring in digital FM modulated transmission systems is the generation of intersymbol interference in the received signal. In general, this is caused by the waveshaping which is required to reduce splatter. When waveshaping is employed, each transmitted bit becomes spread out in time over several adjacent bits resulting in interference during the detection of these bits. Intersymbol interference results in a sensitivity loss in the receiver. Only certain classes of shaped signals are known to exhibit no intersymbol interference. However, most of these shaped signals are extremely complex in structure. A means of adapting one or more of these shaped signals is required if intersymbol interference is to be minimized.
When such complex shaped signals are used, very complicated analog filters are required in the receiver to obtain the low intersymbol interference inherent in the signal. A means is required to adapt these complex shaped signals for use in receivers using conventional post detection filtering commonly found in the FM receivers, such as pagers, which are in wide use today.
Many FM modulation systems also place constraints on the bit rate at which digital data can be transmitted. Any method which reduces splatter while minimizing intersymbol interference should not unduly restrict the maximum bit rate which can be transmitted in a given RF channel.